# [time-nuts] Measuring speed of light or reproducing a metre

Don Latham djl at montana.com
Mon Jun 24 14:03:33 EDT 2013

```IR and acoustic distance sensors already made up with pulse i/o on
breakout boards are available at places like sparkfun (not an ad) or
robot store. In fact, a pair, one acoustic and one ir over the same path
might be very interesting indeed...hmmm
Don

Tom Van Baak
> Jim,
>
> Yes, it makes a very nice demo (I did this as an experiment in college,
> using a hp 5245L).
>
> Set up LED/laser diode, mirror (or other optics), and photo detector so
> that you create an oscillator (it will be many MHz); each pulse received
> generates one pulse out. Measure the frequency. Then simply move the
> mirror by a few dX cm. Again measure the frequency. From this you can
> calculate c.
>
> The beauty of this method is that only numbers you need are dF (=F1-F2)
> and dX. Within reason, all the rest of the factors cancel out; no
> measurement or calibration is required.
>
> Almost everyone of any age knows about canyon echoes, PA system
> feedback, or counting seconds between lightning and thunder. So (like a
> trombone) move the speaker/microphone a few dX feet. Again, all you need
> is dF and dX to calculate the speed of sound.
>
> The speeds differ by almost exactly a factor of a million (sound travels
> about one foot per millisecond; light travels about one foot per
> nanosecond). It should make a stunning audio & visual demo.
>
> /tvb
>
> ----- Original Message -----
> From: "Jim Palfreyman" <jim77742 at gmail.com>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Sent: Sunday, June 23, 2013 4:03 PM
> Subject: [time-nuts] Measuring speed of light or reproducing a metre
>
>
>> Hi all,
>>
>> With a 3325B, a 5370B, and other time-nut miscellany, what's the
>> quickest
>> way you can come up with to measure the speed of light OR reproduce
>> the
>> metre.
>>
>> I've got some ideas, but I'd like others' thoughts.
>>
>> Jim
>
>
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--
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
De Erroribus Medicorum, R. Bacon, 13th century.
"If you don't know what it is, don't poke it."
Ghost in the Shell

Dr. Don Latham AJ7LL
Six Mile Systems LLP